Didn't Na Li just beat Sharapova recently. Maybe she is overly tired and wants to save herself for Wimbledon ? I would say that it is natural for the body to have slight niggles. The body needs time to recover physically and mentally after a full week of competition. Nadal was the exception in earlier years but even he is cutting back on the amount of competition to save his body now.

With Elena injury, again she was the winner just the week before and said that she was very tired after winning the cup. As a general rule, I would say that a person's body needs at least 1 week break after a full week of competition except in a GS where they have a day break in between but would then need two weeks of rest if they get to the final of the GS.